Although many functional chemicals can be added to the wet end of the paper machine as internal sizes, some grades of paper require special properties that cannot be provided by the low levels of additives that are retained at the wet end of the paper machine. An example is a high quality printing and writing grade of paper or paperboard requiring high levels of surface size to provide good printing characteristics, as well as a high surface strength. To achieve the properties required for these grades of paper, it is necessary to apply the chemicals to a preformed paper web, also called surface sizing.
The most common method for the application of chemicals to the surface of a paper web is by a size applicator, such as a size press or a calender water box. In the size press, dry paper is passed through a flooded nip and a solution or dispersion of the functional chemicals contact both sides of the paper. Excess liquid is squeezed out in the press and the paper is redried and cured.
The most commonly used materials for surface sizing of paper and paperboard are water soluble or water dispersible polymers, such as starches and modified starches, polyvinyl alcohols, styrene-maleic anhydride interpolymers and other carboxylated polymers, alkylketene-dimer emulsions, carboxymethyl cellulose, polyurethanes, epoxies and the like, either alone or in mixtures of two or more of these polymers. Other additives such as defoamers, pigments, alkali, and the like are also often added to the treatment solution.
Surface sizing is applied to paper or paperboard to improve various properties of the sheet to render it suitable for the end application. Typical properties imparted by surface size treatment to the paper sheet, after drying and curing, include improved resistance of the surface to moisture, enhanced strength, improved bonding of the cellulosic fibers to prevent subsequent linting, as well as preventing the loss by dusting of the mineral powders that are often added at the wet end of the paper machine to enhance optical properties and also lowering the cost of the final paper sheet. Other important properties of the paper sheet, such as reduced porosity, enhanced ink holdout when printed, and reduction of curl of the sheet can also be achieved by surface sizing.
Surface sizing of paper and paperboard also plays an even more important role when no internal size is used as is often the case, or when certain synthetic internal sizes are used, as is typical for papers made under neutral or alkaline pH conditions. On the other hand, certain synthetic internal sizes, if used at high dosage levels, can cause problems in the operation of the paper machine because of slipperines and hydrolysis of the internal size, and in the reduced quality of the produced paper sheet. These problems can be eliminated by using to the maximum extent possible surface sizing as an alternative to internal sizing. As already mentioned above, surface sizing is applied to beth sides of paper and paperboard.
A major disadvantage limiting the efficiency of a surface size is its tendency to penetrate excessively the paper or paperboard sheet when certain internal sizing agents with slow rates of internal sizing development are used. This reduces the effectiveness of the surface size, because less of the surface size is deposited at the surface of the paper or board sheet, thereby requiring that higher pickup levels be used. It also places more reliance on the internal size to provide sizing levels required of the paper sheet. When salts of carboxylated polymers are used in the surface size in addition to water soluble hydroxylated polymers, i.e., polymers containing hydroxygroups, extra large addition levels are needed to compensate for this penetration. The reduced concentration of the surface sizing compound at the surface of the sheet can result in paper sheet problems. Other problems are caused by the large amount of carboxylated polymers necessary to overcome the effect of penetration into the sheet, a common problem being the generation of foam. Foam reduces pickup of the surface sizing compound, causes defects on the paper surface, and interferes with the efficient operation of the paper machine. Carboxylated polymers are also more expensive than hydroxylated polymers, such as starch, and their use should be minimized for that reason.
There has been found a way to improve the surface holdout of the surface size by adding a group IV of the periodic system of elements metal salt, for example ammonium zirconium carbonate (AZC) to the surface sizing compound to maximize its effect. Other group IV metal salts useful in the instant invention are those of hafnium and titanium. Zirconium salts have previously been suggested, for example, in U.S. Pat. No. 4,400,440 issued to Shaw, as well as in Brit. Patent 1,024,881 issued to the Inveresk Paper Company, to impart property improvements when used in conjunction with emulsion polymers for significantly improved block resistance of a pigmented coating composition that was heated and cured to crosslinking the coating binder. Zirconium salts have also been suggested as migration inhibitors for non-woven binders as taught in U.S. Pat. No. 3,930,074 issued to Drelich.